The present invention relates to a multi-purpose dielectric resonator filter for use at a mobile communication base station to serve as each of a receiving filter, a transmitting filter, a duplexer, and the like.
Conventionally, band pass filters for allowing the passage of only signals in a specified frequency band have been used at base stations for mobile communication such as a mobile phone. For example, a receiving system uses a receiving filter to remove signals for communication systems using the other frequency bands and a transmitting system uses a transmitting filter not to send undesired electric waves to the systems using the other frequency bands. Such filters for use at the base stations are required to have a sufficiently low loss to provide the base stations with an adequate receiving sensitivity and power efficiency, a sharp filter characteristic provided for a reduced interval in frequency band between the adjacent base stations, and reduced size and weight for easier mounting on the overheads of the base stations. As an example of a filter satisfying such requirements, a dielectric resonator filter composed of a plurality of dielectric resonators coupled to each other has been proposed, which comes in various configurations.
FIG. 21 is a perspective view schematically showing an example of a conventional six-stage dielectric resonator filter. As shown in FIG. 21, the conventional dielectric resonator filter comprises six cylindrical dielectric resonators 511A to 511F formed by sintering a dielectric powder material. The resonance frequency of each of the dielectric resonators 511A to 511F is determined by the height and diameter of the cylindrical configuration thereof. In this example, the six dielectric resonators 511A to 511F operate as a six-stage band pass filter. An enclosure 520 of the dielectric resonator filter comprises a main body 521 composed of a bottom wall and side walls, a lid 522, partition walls 523A to 523G connected to each other to partition, into chambers, a space enclosed by the enclosure main body 521. The dielectric resonators 511A to 511F are disposed on a one-by-one basis in the respective chambers defined by the partition walls 523A to 523G of the enclosure 520. Interstage-coupling tuning windows 524A to 524E for providing electromagnetic field couplings between the resonators are provided between the five partition walls 523A to 523E of the seven partition walls 523A to 523G and the side walls of the enclosure main body 521. The interstage-coupling tuning windows 524A to 524E are provided with respective interstage-coupling tuning bolts 531A to 531E each for tuning the strength of an electromagnetic field coupling between the resonators. The enclosure main body 521 is provided with input/output terminals 541 and 542 each composed of a coaxial connector to input and output a high-frequency signal to and from the outside. Input/output coupling probes 551 and 552 are connected to the respective core conductors of the input/output terminals 541 and 542.
Resonance-frequency tuning members 561A to 561F each composed of a disk and a bolt formed integrally to tune the resonance frequency of the corresponding one of the dielectric resonators 511A to 511F are attached to the enclosure lid 521. The resonance-frequency tuning members 561A to 561F are disposed to have their respective center axes at the same plan positions as the respective center axes of the dielectric resonators 511A to 511F (i.e., at the concentric positions).
Since the frequency characteristics including passband width and attenuation characteristic of a dielectric resonator filter are generally determined by the resonance frequency and Q factor of each of the resonators and an amount of coupling between the individual dielectric resonators, the configuration and the like of each of the dielectric resonators are calculated from the specifications of the frequency characteristics of the filter at the design stage. In practice, however, filter characteristics as designed cannot be obtained due to an error in the configurations of the dielectric resonators and enclosure and to a mounting error. To provide filter characteristics as designed, the resonance-frequency tuning members 561A to 561F are provided in the conventional dielectric resonator filter to render the respective resonance frequencies of the dielectric resonators 511A to 511F variable. In addition, the interstage-coupling tuning bolts 531A to 531E are provided to render the strengths of interstage couplings variable. Through the tuning using the tuning mechanism, desired filter characteristics are provided.
For the resonance-frequency tuning members 561A to 561F, a structure as shown in FIG. 21 has been used widely in which the frequency characteristics of the dielectric resonators 511A to 511F are made variable by tuning the distance between conductor plates opposed to the dielectric resonators 511A to 511F and the dielectric resonators 511A to 511F by using the bolts.
The dielectric resonator filter having such a structure operates as follows. If a high-frequency signal transmitted from, e.g., a signal source or an antenna and inputted into the enclosure 520 via the input/output terminal 541 has a frequency within the pass band of the filter, the signal couples to an electromagnetic field mode in the input-stage dielectric resonator 511A by the effect of the input/output coupling probe 551 so that TE01 δ as a basic resonance mode is excited.
The resonance mode couples to respective electromagnetic field modes in the subsequent dielectric resonators 511B, 511C, . . . in succession through the interstage-coupling tuning windows 524A, 524B, . . . so that the electromagnetic field mode excited in the dielectric resonator 511F couples to the output-side input/output probe 552 and the high-frequency signal is outputted from the input/output terminal 542. On the other hand, the high-frequency signal having a frequency outside the pass band of the filter is reflected without coupling to the resonance mode in the dielectric resonator and sent back from the input/output terminal 541.
FIG. 24 is a perspective view schematically showing an example of a conventional four-stage dielectric resonator filter. As shown in FIG. 24, the conventional dielectric resonator filter comprises four cylindrical dielectric resonators 611A to 611D formed by sintering a dielectric powder material. In this example, the four dielectric resonators 611A to 611D operate as a four-stage band pass filter. An enclosure 620 of the dielectric resonator filter comprises a main body 621 composed of a bottom wall and side walls, a lid 622, and partition walls 623A to 623D connected to each other to partition, into chambers, a space enclosed by the enclosure main body 621. The dielectric resonators 611A to 611D are disposed on a one-by-one basis in the respective chambers defined by the partition walls 623A to 623D of the enclosure 620. Interstage-coupling tuning windows 624A to 624C for providing electromagnetic field couplings between the resonators are provided between the three partition walls 623A to 623C of the four partition walls 623A to 623D and the side walls of the enclosure main body 621. The interstage-coupling tuning windows 624A to 624C are provided with respective interstage-coupling tuning bolts 631A to 631C each for tuning the strength of an electromagnetic field coupling between the resonators. The enclosure main body 621 is provided with input/output terminals 641 and 642 each composed of a coaxial connector to input and output a high-frequency signal to and from the outside. Input/output coupling probes 651 and 652 are connected to the respective core conductors of the input/output terminals 641 and 642.
Resonance-frequency tuning members 661A to 661D each composed of a disk and a bolt formed integrally to tune the resonance frequency of the corresponding one of the dielectric resonators 611A to 611D are attached to the enclosure lid 621. The resonance-frequency tuning members 661A to 661D are disposed to have their respective center axes at the same plan positions as the respective center axes of the dielectric resonators 611A to 611D (i.e., at the concentric positions).
However, the foregoing conventional dielectric resonator filters have the following drawbacks.
FIG. 23 shows an example of the frequency characteristic of the dielectric resonator filter shown in FIG. 21. In FIG. 23, the horizontal axis represents the frequency. (GHz) and the vertical axis represents the transmission characteristic (dB). As can be seen from the drawing, an attenuation pole P1 (valley) with an enhanced transmission characteristic exists in the pass band, which indicates that the filter characteristic has been degraded. The present inventors have assumed the cause of such a degraded filter characteristic as follows.
FIG. 22 shows an electromagnetic field mode in the vicinity of the conductor plate of each of the resonance-frequency tuning members 561 of the dielectric resonator filter shown in FIG. 21. In the drawing is shown the result of analyzing the distribution of an electric field in a cross section passing through the axis of the resonance-frequency tuning member by an electromagnetic field simulation using a FDTD method. As shown in FIG. 22, a spurious electromagnetic field mode is produced in a space defined by the conductor plate of the resonance-frequency tuning member 561 and the enclosure lid 522.
As a result, the spurious electromagnetic field mode couples to a high-frequency signal to cause the state of resonance so that the spurious attenuation pole P1 (valley portion) is assumed to appear in the frequency characteristic. The spurious mode reacts more sensitively to the movement of the resonance-frequency tuning member than the resonance frequency in a basic mode required to provide the filter characteristic and changes greatly. Consequently, the attenuation pole resulting from the spurious mode frequently passes through a near-passband region when the vertical position of the resonance-frequency tuning member is changed to tune the filter characteristic and disturb the waveform of the filter characteristics, which presents a large obstacle to the tuning operation. In the worst case, the spurious mode enters the pass band of the filter even after the resonance-frequency tuning operation is completed to degrade the filter characteristic, as shown in FIG. 23.
In addition, the conventional dielectric resonator filters have the problem that a coupling between high-order modes different from the basic resonance mode in the dielectric resonators causes an undesired harmonic component at frequencies higher than the pass band of the filter. In principle, a component at a frequency higher than the pass band is removed by a low pass filter. However, there is an upper limit to the level of a signal that can be removed by the low pass filter. Therefore, strict specifications have been determined for the harmonic component in addition to the specifications of the pass band of a filter used at a base station of a mobile phone to suppress the level of the harmonic component.
FIG. 25 shows an example of the frequency characteristic of the conventional four-stage dielectric resonator filter. As shown in the drawing, a harmonic component on a level that cannot be removed completely by a low pass filter (e.g., −40 dB or more) may be produced in the conventional dielectric resonator filter. The present inventors have considered that the cause thereof is an insufficient capability of tuning the interstage couplings.